The leading Republican plan to replace Obamacare could cause health plan premiums to rise by 30 percent or more in 2018 — and even higher in future years, an insurance company CEO said Tuesday.

J. Mario Molina also warned in a Wall Street Journal interview that enrollment in Obamacare plans could decrease by 75 percent or more, with the remaining customers being sicker overall

"You're going to see big rate increases, and you're going to see insurers exit markets," the chief executive of Molina Healthcare told the Journal.

"This is going to destabilize the marketplace," said Molina, whose company sells Obamacare plans in nine states.

Molina says the big rate hikes next year would come as a result of the GOP plan to kill the Obamacare mandate that requires most people to have some form of health coverage or pay a tax penalty.

That move would lead to many healthy people deciding not to buy insurance.

Bigger rate hikes, Molina said, would come in future years, once the Republican plan's tax credits take effect.

Those tax credits are less generous than the subsidies that most Obamacare customers currently receive to help reduce the cost of their monthly premiums.

Molina's comments led Andy Slavitt, who previously oversaw Obamacare for the Obama administration as head of the Centers for Medicare and Medicaid Services to tweet that the GOP plan was dead on arrival.